Updating Canvas? invalidate()?

Question

Hi im trying to get my app to update a canvas from a custom view i made This view creates a square with lines and a circle in the center. I want to press a button and draw random x and y coordinates to the canvas.

Heres My MainActivity:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); 
//  EditText numDart = (EditText) findViewById(R.id.numDarts);

    setContentView(R.layout.activity_main);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

public int convertToDpi(int px){
    DisplayMetrics metrics = getResources().getDisplayMetrics();
    int unit = metrics.widthPixels/20;
    return px * unit;
}
public void drawCanvas(View v){
    View view = (View) findViewById(R.id.canView);
    Paint black = new Paint();
    black.setColor(Color.BLACK);
    black.setStyle(Style.FILL);

    view.invalidate(); //dont know where to go from here
}

Heres My Custom View:

public CanView(Context context, AttributeSet attrs) {
    super(context, attrs);
    // TODO Auto-generated constructor stub
}

@Override
protected void onDraw(Canvas canvas) {
    // TODO Auto-generated method stub
    super.onDraw(canvas);

    Rect myrect = new Rect();
    myrect.set(-10, 10, 10, -10);

    Paint blue = new Paint();
    Paint white = new Paint();
    white.setColor(Color.WHITE);
    white.setStyle(Paint.Style.FILL);

    Paint black = new Paint();
    black.setColor(Color.BLACK);
    black.setStyle(Paint.Style.FILL);

    blue.setColor(Color.BLUE);
    blue.setStyle(Paint.Style.FILL);


    canvas.drawRect(myrect, blue);
    canvas.drawCircle(convertToDpi(10), convertToDpi(10),convertToDpi(3), white);
    canvas.drawLine(convertToDpi(10), 0, convertToDpi(10), convertToDpi(20), black);
    canvas.drawLine(0, convertToDpi(10), convertToDpi(20), convertToDpi(10), black);
    canvas.scale(5, 5, 0, 0);
}
@Override
public void postInvalidate() {  //Logic for redrawig goes here??? 
    // TODO Auto-generated method stub
    super.postInvalidate();
}

public int convertToDpi(int px){
    DisplayMetrics metrics = getResources().getDisplayMetrics();
    int unit = metrics.widthPixels/20;
    return px * unit;
}

I dont get how to reference the canvas from my Custom View and change or redraw it. Im assuming you use invalidate(); but im puzzled on how this method works can any1 help me out?

Answer 1

When invalidate is called onDraw is called again, you should also use this.getHolder().addCallback(this); and let your class implement Callback and add the unimplemented methods